Tellurium-containing nanocrystalline materials

ABSTRACT

Tellurium-containing nanocrystallites are produced by injection of a precursor into a hot coordinating solvent, followed by controlled growth and annealing. Nanocrystallites may include CdTe, ZnTe, MgTe, HgTe, or alloys thereof. The nanocrystallites can photoluminesce with quantum efficiencies as high as 70%.

CLAIM OF PRIORITY

This application claims priority to U.S. patent application Ser. No.60/145,708, filed on Jul. 26, 1999, and is a continuation-in-part ofU.S. patent application Ser. No. 08/969,302, filed Nov. 13, 1997, nowU.S. Pat. No. 6,322,901, each of which is hereby incorporated byreference in its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.DMR-94-00334 from the National Science Foundation. The government hascertain rights in the invention.

TECHNICAL FIELD

The invention relates to tellurium-containing nanocrystalline materials,and to methods for making such materials.

BACKGROUND

Semiconductor nanocrystallites having radii smaller than the bulkexciton Bohr radius constitute a class of materials intermediate betweenmolecular and bulk forms of matter. Quantum confinement of both theelectron and hole in all three dimensions leads to an increase in theeffective band gap of the material with decreasing crystallite size.Consequently, both the optical absorption and emission ofnanocrystallites shift to the blue (i.e., to higher energies) as thesize of the crystallite gets smaller.

Bawendi and co-workers have described a method of preparing monodispersesemiconductor nanocrystallites by pyrolysis of organometallic reagentsinjected into a hot coordinating solvent (J. Am. Chem. Soc., 115:8706(1993)). This permits temporally discrete nucleation and results in thecontrolled growth of macroscopic quantities of nanocrystallites. Theparticle size distribution can be refined by size selectiveprecipitation. The narrow size distribution of nanocrystallites canallow the particles to have narrow spectral width emissions. Thesetechniques can yield excellent results in the production ofselenium-containing II-VI semiconductor nanocrystallites.

SUMMARY

The invention provides methods of synthesizing telluride semiconductornanocrystallites. The nanocrystallites can have high quantumefficiencies and can have narrow size distributions. The telluridesemiconductors have relatively smaller band gaps than their selenide andsulfide analogs, and can expand the range of colors available usingII-VI photoluminescent nanocrystallites further into the far red rangeof the spectrum. In particular, cadmium telluride nanocrystallites canemit in wavelengths sufficiently long to make them suitable for use inmulticolor detection schemes for whole blood diagnostics, where emissionwavelengths of at least 630 nm can be preferred.

In one aspect, the invention features a nanocrystallite including a coreof MTe, where M is cadmium, zinc, magnesium, mercury, or mixturesthereof. The core can have an overcoating on a surface of the core. Theovercoating can be a semiconductor having a composition different fromthe core.

In another aspect, the invention features a nanocrystallite includingMTe that can photoluminesce with a quantum efficiency of at least 20%.

In another aspect, the invention features a method of manufacturingnanocrystallites by injection of an M-containing compound, M beingcadmium, zinc, magnesium, mercury, or mixtures thereof, and aTe-containing compound of the form

where at least one of Z, Z′, and Z″ is an amide. Preferably, two of Z,Z′, and Z″ are, independently, amides, and more preferably three of Z,Z′, and Z″ are, independently, amides. The mixture is heated to grow thenanocrystallites. The heating can be controlled in such a way that thegrowth is controlled. The M-containing compound and a Te-containingcompound can be premixed, or M and Te can be incorporated into differentpositions of a single molecule. The M-containing compound and theTe-containing compound can be injected sequentially or simultaneously.Additional M-containing compound, additional Te-containing compound, ora mixture thereof, can be added to the mixture during heating. Anovercoating can be grown on a surface of the nanocrystallite. Thenanocrystallites can be separated by size selective precipitation. Anamine can be added to the mixture during size selective precipitation.The Te-containing compound can include a tris(dialkylamino)phosphinetelluride. The Te-containing compound can have a boiling point of atleast 200° C., preferably 250° C., and more preferably 280° C., at oneatmosphere.

In another aspect, the invention features a Te-containing compound ofthe form

where at least one of Z, Z′, and Z″ is an amide, and a method ofpreparing a Te-containing compound including contacting P(Z)(Z′)(Z″)with Te.

The nanocrystallite can have a quantum efficiency of emission of atleast 30%, 40%, 50%, 60%, or 70%. The quantum efficiency can be as highas 75%, 80%, 90%, 95% or 99%. The quantum efficiency can be between 20and 99%, preferably between 30 and 99%, more preferably between 40 and95%, and most preferably between 70 and 95%. The nanocrystallite can bea member of a size-selected population having no more than a 15% RMSdeviation from mean diameter, preferably 10% RMS deviation or less, andmore preferably 5% RMS deviation or less. CdTe nanocrystallites canphotoluminesce and can have emission wavelengths in the range of 580 to770 nm, preferably 550 to 780 nm, and more preferably 435 to 800 nm. Thenanocrystallite can photoluminesce with a full-width at half maximum(FWHM) of 70 nm or less, preferably 45 nm or less, more preferably 20 nmor less, and most preferably 15 nm or less for a single nanocrystallite.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1(a) is a diagram depicting a method according to the invention.

FIG. 1(b) is a drawing depicting structures of two precursors forsynthesis of CdTe nanocrystals.

FIGS. 2(a)-2(c) are graphs depicting the UV/vis absorption spectra andthe x-ray diffraction spectra of CdTe nanocrystals prepared with TOPTeand HPPTTe.

FIGS. 3(a) and 3(b) are graphs depicting the development of the UV/visabsorption spectra during nanocrystallite synthesis.

FIG. 4 is a graph depicting the evolution of photoluminescence duringnanocrystallite synthesis.

FIG. 5 is a graph depicting shows the accessible absorption spectra forCdTe nanocrystallites.

FIG. 6 is a graph depicting the evolution of the photoluminescence of adilute solution of nanocrystallites exposed to air.

FIG. 7 is a graph depicting the evolution of the photoluminescence of adilute solution of nanocrystallites in an air-free environment.

FIG. 8 is a graph depicting the intensity lifetimes of CdTenanocrystallites stored under air and under nitrogen.

FIG. 9 is a graph depicting the evolution of the photoluminescence as afunction of temperature.

FIG. 10 is a TEM image of telluride semiconductor nanocrystallites.

FIG. 11 is a series of graphs depicting size histograms for severalnanocrystallite syntheses.

FIG. 12 is a graph depicting the dependence of the photoluminescence ondiameter for CdTe nanocrystallites.

FIG. 13 is a graph depicting x-ray diffraction spectra for three sizesof CdTe nanocrystallites.

FIG. 14 is a graph depicting the band gaps of several bulk II-VIsemiconductors.

FIG. 15 is a graph depicting the emission spectra of a series of sizesof CdTe nanocrystallites.

DETAILED DESCRIPTION

Tellurium-containing nanocrystallites can be produced by injection of anM-containing compound and a Te-containing compound into a hotcoordinating solvent, followed by growth and annealing of thenanocrystallites. The nanocrystallites can include CdTe, ZnTe, MgTe,HgTe, or alloys thereof. By proper selection of precursor compositionand stoichiometry, telluride semiconductor photoluminescentnanocrystallites having quantum efficiencies as high as 70% can beproduced.

Improved telluride nanocrystallites can be produced by varying theprecursor compounds and the precursor stoichiometry from that describedin U.S. patent application Ser. No. 08/969,302, filed Nov. 13, 1997,which is incorporated herein by reference in its entirety. Cadmiumtelluride nanocrystallites made by the methods described in U.S.application Ser. No. 08/969,302 using dimethyl cadmium (Me₂Cd) andtrioctylphosphine telluride (TOPTe) as precursors exhibit inefficientphotoluminescence, having quantum efficiencies of less than 1%. Thetelluride-containing nanocrystallites produced using a Te-containingcompound including an amino group can be prepared having quantumefficiencies as high as 70%, more than threefold higher than the quantumefficiency of 20% reported for colloidal CdTe nanocrystallites (J. Phys.Chem. 1993(97):11999-12003). The quantum efficiency of nanocrystallitescan be further enhanced by overcoating a core nanocrystallite with alayer of a second semiconductor material (e.g., ZnS or ZnSe overcoatedCdTe cores).

Tellurium-containing nanocrystallites are obtained using a hightemperature colloidal growth process, preferably followed by sizeselective precipitation. The high temperature colloidal growth processcan be accomplished by rapidly injecting an appropriate combination ofan M-containing compound and Te-containing compound into a hotcoordinating solvent to produce a temporally discrete homogeneousnucleation and controlling the growth of the nuclei intonanocrystallites. Injection of reagents into the hot reaction solventresults in a short burst of homogeneous nucleation. This temporallydiscrete nucleation is attained by a rapid increase in the reagentconcentration upon injection, resulting in an abrupt supersaturation,which is relieved by the formation of nuclei and followed by growth onthe initially formed nuclei. The partial depletion of reagents throughnucleation and the sudden temperature drop associated with theintroduction of room temperature reagents prevents further nucleation.

The solution then may be gently heated to reestablish the solutiontemperature. Gentle reheating allows for growth and annealing of thenanocrystallites. The higher surface free energy of the smallcrystallites makes them less stable with respect to dissolution in thesolvent than larger crystallites. The net result of this stabilitygradient is the slow diffusion of material from small particles to thesurface of large particles (“Ostwald ripening”). In addition, thereagents remaining in the coordinating solvent may contribute to growth;this effect may be encouraged by feeding additional reagents to thesolution during growth. Growth and ripening of this kind result in ahighly monodisperse colloidal suspension from systems which mayinitially be highly polydisperse. The process of slow growth andannealing of the nanocrystallites in the coordinating solvent thatfollows nucleation results in uniform surface derivatization and regularcore structures. Both the average size and the size distribution of thecrystallites in a sample are dependent on the growth temperature. Thegrowth temperature necessary to maintain steady growth increases withincreasing average crystal size. As the size distribution sharpens, thetemperature may be raised to maintain steady growth. The growth periodmay be shortened significantly by using a higher temperature or byadding additional precursor materials. The overall process is shownschematically in FIG. 1(a).

Size distribution during the growth stage of the reaction can beestimated by monitoring the absorption line widths of the particles.Modification of the reaction temperature in response to changes in theabsorption spectrum of the particles allows the maintenance of a sharpparticle size distribution during growth. Reactants can be added to thenucleation solution during crystal growth to grow larger crystals. Thephotoluminescence (PL) spectra of the nanocrystallites can be tunedcontinuously over maximum emission wavelengths from 550 to 780 nm,complementing the available wavelengths for nanocrystallites having CdSecores. The wavelength of maximum emission can be tuned by stopping thegrowth a particular average size of nanocrystallite. The particle sizedistribution may be further refined by size selective precipitation.

The M-containing compound can be an organometallic compound, such as analkyl-M compound. For example, the M-containing compound can be MRQwherein M is Cd, Zn, Hg or Mg, and R and Q are, independently, alkyl,alkenyl, aryl, cycloalkyl, or cycloalkenyl. Preferred examples includedialkyl Cd, dialkyl Zn, dialkyl Hg or dialkyl Mg.

The Te-containing compound can be a stable phosphine telluride,preferably a tris amido phosphine telluride. The Te-containing compoundcan have formula

At least one of Z, Z′, and Z″ can be an amide. Preferably, two of Z, Z′,and Z″ are amides. The remaining groups of Z, Z′, and Z″ can be alkyl,alkenyl, aryl, cycloalkyl, or cycloalkenyl, or derivatives thereof. Morepreferably, each of Z, Z′, and Z″ is an amide. Each of Z, Z′, and Z″ canhave the formula —N(A)(A′), where each of A and A′, independently, isalkyl, alkenyl, aryl, cycloalkyl, or cycloalkenyl, or a derivativethereof. Preferably, each of Z, Z′, and Z″ is a dialkyl amide. Eachalkyl can be a lower alkyl. The Te-containing compound has a boilingpoint of at least 200° C. One suitable Te-containing compound ishexapropylphosphorustriamide telluride (HPPTTe). HPPTTe producesunexpectedly high quantum efficiency nanocrystallites as comparednanocrystallites prepared from trioctylphosphine telluride (TOPTe). Thestructures of HPPTTe and TOPTe are shown in FIG. 1(b).

Alternatively, a single compound containing M and Te can be used as boththe M-containing compound and the Te-containing compound. An example ofsuch a compound is a Te-containing compound in which one of Z, Z′, andZ″ includes a dialkyl M moiety.

Alkyl is a branched or unbranched saturated hydrocarbon group of 1 to100 carbon atoms, preferably 1 to 30 carbon atoms, such as methyl,ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, octyl, decyl,tetradecyl, hexadecyl, eicosyl, tetracosyl and the like, as well ascycloalkyl groups such as cyclopentyl, cyclohexyl and the like. The term“lower alkyl” includes an alkyl group of 1 to 20 carbon atoms,preferably 2 to 8 carbon atoms.

Alkenyl is a branched or unbranched hydrocarbon group of 2 to 100 carbonatoms containing at least one carbon-carbon double bond, such asethenyl, n-propenyl, isopropenyl, n-butenyl, isobutenyl, t-butenyl,octenyl, decenyl, tetradecenyl, hexadecenyl, eicosenyl, tetracosenyl andthe like. The term “lower alkenyl” includes an alkenyl group of 2 to 20carbon atoms, preferably 2 to 8 carbon atoms, containing one —C═C— bond.

Optionally, an alkyl, or alkenyl chain can contain 1 to 6 linkagesselected from the group consisting of —O—, —S—, —M— and —NR— where R ishydrogen, lower alkyl or lower alkenyl.

Aryl is a monovalent aromatic hydrocarbon radical consisting of one ormore fused rings in which at least one ring is aromatic in nature, whichcan optionally be substituted with one or more of the followingsubstituents: hydroxy, cyano, alkyl, alkoxy, thioalkyl, halo, haloalkyl,hydroxyalkyl, nitro, amino, alkylamino, and dialkylamino, unlessotherwise indicated.

Cycloalkyl is reference to a monovalent saturated carbocyclic radicalconsisting of one or more rings, which can optionally be substitutedwith one or more of the following substituents: hydroxy, cyano, alkyl,alkoxy, thioalkyl, halo, haloalkyl, hydroxyalkyl, nitro, amino,alkylamino, and dialkylamino, unless otherwise indicated.

Cycloalkenyl includes reference to a monovalent unsaturated carbocyclicradical consisting of one or more rings and containing one or morecarbon-carbon double bonds, which can optionally be substituted with oneor more of the following substituents: hydroxy, cyano, alkyl, alkoxy,thioalkyl, halo, haloalkyl, hydroxyalkyl, nitro, amino, alkylamino anddialkylamino, unless otherwise indicated.

The absolute quantum efficiency of the nanocrystallites was as high as70%. The average quantum efficiency of a group of 15 nanocrystallitesamples was 60±5%, attesting to the reproducibility of the method. FIG.2 displays UV/vis absorption spectra from CdTe preparations using (a)TOPTe and (b) HPPTTe. Initial absorption spectra from each reaction showsimilar qualitative features. Nucleation produces a bimodal distributionof sizes, with the smaller species quickly dissolving into feedstock forthe larger nanocrystallites. After three hours of stirring at 290° C.,each reaction has effectively the same absorption spectrum. As monitoredby absorption spectroscopy, both reactions appear the same. However, thequantum efficiency of the CdTe synthesized with TOPTe is only 1%,whereas the quantum efficiency of CdTe produced with HPPTTe is 55%.

Without wishing to be bound by any particular explanation, it isbelieved that the strong electron-donating properties of the amidegroups in the HPPTTe are at least partially responsible for theimprovement in nanocrystal quality. Te-containing compounds having oneor two amide groups can produce significantly improved quantumefficiencies for telluride nanocrystallites compared to trialkylphosphine telluride compounds. A suitable Te-containing compound shouldbe stable at reaction temperature (e.g., having a boiling point of atleast 200° C.).

The interchangeability of the spectra of FIGS. 2(a) and 2(b) suggeststhat UV/vis absorption spectroscopy is not sensitive to the differenceswhich would explain the PL intensities. TEM and XRD analysis also didnot reveal any substantial differences between the two species, as shownby the XRD spectra in FIG. 2(c).

The nanocrystallites can be overcoated with a coating of a semiconductormaterial. For example, ZnS, ZnSe or CdSe overcoatings can be grown onCdTe nanocrystallites.

The telluride nanocrystallites can be suitable for a variety ofapplications, including those disclosed in copending and commonly ownedU.S. patent application Ser. Nos. 09/156,863, filed Sep. 18, 1998,09/160,454, filed Sep. 24, 1998, 09/160,458, filed Sep. 24, 1998, and09/350,956, filed Jul. 9, 1999, all of which are incorporated herein byreference in their entirety. In particular, a light-emitting device asdescribed in Example 3 of U.S. patent application Ser. No. 09/350,956constructed using CdTe nanocrystallites produces a very intense redlight.

EXAMPLES Synthesis of CdTe Nanocrystallites

Unless otherwise noted, all reactions were carried out in a dry nitrogenatmosphere using a glovebox or standard Schlenk techniques. HPLC gradesolvents used for size selective precipitation were purged of dissolvedoxygen by bubbling with nitrogen for 5 minutes. Trioctylphosphine (TOP,95%) was used as received from Fluka. Tellurium shot (99.999%, lowoxide) was used as received from Alfa/Aesar. Hexa-n-propyl phosphoroustriamide (HPPT, 97%, Lancaster) was vacuum distilled, collecting thefraction boiling between 83-103° C. at 0.55 Torr. Trioctylphosphineoxide (TOPO, 90%, Strem) was dried under vacuum (˜0.5 Torr) for 1 hour.Dimethyl cadmium (99+%, Strem) was purified by vacuum transfer.

A stock solution of hexapropylphosphorustriamide telluride (HPPTTe) wasprepared by adding 6.38 g tellurium shot to 45.00 g HPPT and stirringuntil dissolved (1-2 days). 20 g TOPO was dried under vacuum (˜0.5 Torr)at 180° C. for 1 hour, then filled with N₂ and heated to 350° C. In a N₂atmosphere glovebox a solution containing 50 μL CdMe₂ (0.69 mmol), 0.35mL HPPTTe stock (0.35 mmol), and 12 mL TOP was mixed very well andloaded into a syringe. This solution was smoothly injected (˜0.5 sec)into the vigorously stirring TOPO, which immediately turned red andcooled to 270° C. When the reaction solution was sampled within 20seconds of injection, the initial UV/vis spectrum displayed a bimodaldistribution of sizes, with absorption features at 435 and 560 nm.Subsequent absorption spectra showed no evidence of a high energy peak,with the first absorption feature now peaked at 580 nm. The temperaturewas raised to 290° C. and the sample was grown to the desiredwavelength. The flask was then cooled to ˜60° C. and mixed with 10 mLbutanol. This solution can be stored (under nitrogen) for at least 6months without noticeable decrease in quantum efficiency.

Nanocrystallites were isolated in air by a modified size selectiveprecipitation with acetonitrile. The reaction solution prepared abovewas mixed with an additional 10 mL butanol. Acetonitrile was added untilthe mixture became turbid. Upon sitting for a few minutes the solutionseparated into two layers. The colorless hydrophilic phase was thenremoved and discarded. The clear red hydrophobic phase was mixed withapproximately one third its original volume of butanol. The process ofadding acetonitrile until turbidity and separating layers was repeateduntil a powder or very thick oil was obtained. Freshly prepared CdTenanocrystallites isolated in this fashion are moderately soluble inhexane and extremely soluble in tetrahydrofuran (THF). Addition of asmall amount of TOP (˜1% vol) helped preserve the luminescence intensityof the size selected material. Yields of crude CdTe ranged from 50 mg(small sizes) to 75 mg (large sizes) of dry powder.

CdTe nanocrystallites were synthesized using a method that employed a 1Mhexapropyl phosphorous triamide telluride (HPPTTe) solution as thechalcogenide-containing compound. Distillation of the HPPT before usewas found to significantly increase the quantum efficiency of thesamples. Purification of CdMe₂ by vacuum transfer gave more consistentresults than filtration through 0.2 μm PTFE membrane. Also, a cadmiumrich preparation was found to give better results. The optimum Cd:Teinjection solution ratio for the conditions of this example wasdetermined empirically to be 2:1. FIG. 3 displays UV/vis absorptionspectra at various times during CdTe synthesis. FIG. 3(a) highlights theearly stages of the synthesis. The bottom spectrum of FIG. 3(a), taken10 seconds after injection when the temperature had fallen to 280° C.,shows that the reaction solution contained a bimodal distribution ofsizes. Well defined features at 2.23 eV (566 nm) and 2.84 eV (437 nm)are clearly visible. A species absorbing at or near 435 nm can besynthesized by performing the injection at 200° C. or below. These arethe smallest nanocrystallites to display a nanocrystallite-likeabsorption and appear to be the 435 nm-absorbing CdTe species equivalentto 410 nm absorption CdSe. Two minutes into the reaction theconcentration of this 435 nm-absorbing CdTe species was greatly reduced,and at 7 minutes there was no trace of this species. These 435nm-absorbing CdTe species did not appear to be growing. No spectralfeature has been observed between 435 and 550 nm during reactionsperformed under similar initial conditions. Instead, the 435nm-absorbing CdTe species appeared to be dissolving as the absorption ofthe larger nanocrystallites grew and sharpened. Growth of the largerparticles at the expense of the smaller ones is an Ostwald ripeningmechanism and is to be expected given the difference in size (˜35 Åversus ˜15 Å). The observation of this mechanism would not benoteworthy, except for the fact that in this case the dissolving speciesis the 435 nm-absorbing CdTe species . The other semiconductor materialsin this family, CdS and CdSe, do not show similar behavior in theirgrowth pattern.

Characterization of Nanocrystallites

UV/visible absorption spectroscopy of CdTe nanocrystallites in hexanewas performed on an HP 8453 diode array spectrometer with 1 nmresolution. Fluorescence measurements were made using a SPEX Fluorolog-2spectrofluorometer which consisted of two double monochromators with2400 grooves/inch gratings blazed at 500 nm and photomultiplier tube(R928) detector. CdTe samples were dissolved in either hexane or THFwith ˜1% TOP in a 1 cm quartz cuvette and diluted until the absorbanceof the first feature was below 9.3. Spectra were obtained in front facegeometry and were corrected for the wavelength dependence of the opticsand detector by multiplication with a suitable correction factor file(MCOR1097.SPT).

FIG. 3(b) displays absorption spectra of aliquots taken from a singlesynthesis of CdTe. A well defined first absorbing state is visible inall spectra, indicating that the growth is controlled. FIG. 4 shows thecorresponding photoluminescence (PL) emission spectra for the samereaction. Spectra have been standardized relative to a methanol solutionof rhodamine 640 (quantum efficiency=100%) so as to accurately reflectthe emission intensity at various points during the reaction, arelationship which is plotted in the inset FIG. 4(b). Three featuresstand out in FIG. 4. First, the quantum efficiency improved over time.This result suggests the importance of an annealing effect. For smallnanocrystallites it can be experimentally difficult to separate thermalannealing from particle growth. Second, all emission occurred at theband edge. There was no low energy (or deep trap) light detected forthis size range. The spectral window was examined down to 1.18 eV (1050nm) using a CCD detector without observing deep trap emission. Moreimportantly, CdTe samples as small as ˜35 Å diameter displayed no deeptrap emission, whereas similarly sized CdS and CdSe luminescence spectragenerally contain at least 20% deep trap emission. The third aspect ofthe emission is simply the sheer magnitude of the quantum efficiency.The nanocrystals used to generate FIG. 4 reaches 55%, but samples ashigh as 70% have been prepared.

FIG. 5 presents absorption spectra illustrating the range of CdTe sizesthat have been produced by this method. Any size/energy between the twoextremes can be produced. Slight adjustments in starting materialconcentration and/or reaction temperature should enable the lowerdiameter limit to be reduced below its present value of ˜44 Å.

The stability with respect to flocculation and PL intensity of dilutesolutions of CdTe nanocrystallites were qualitatively less than those ofCdSe because the CdTe nanocrystallites are air sensitive. FIG. 6 showsthe PL of a dilute hexane solution of 53 Å diameter CdTe as the emissiondecreases over time in air. The insert of FIG. 6 plots intensity versustime over the entire experiment. The emission intensity is effectivelyzero after 2.5 hours. At this point the solution is also quite turbid. Acomparative experiment was conducted under nitrogen and the results areshown in FIG. 7. Even after 39 hours the fluorescence intensity has onlyfallen to just under half the initial value. Also, after 12 hours theair-free solution becomes very turbid. The solution phase “lifetimes” ofthe PL intensities are compared in FIG. 8. Results presented here showthat the useable lifetime of CdTe nanocrystallites, particularly indilute solutions, can be greatly extended by keeping thenanocrystallites in an inert atmosphere. While manipulating moreconcentrated solutions in air, for example, during size selectiveprecipitation, PL does not appear to be greatly diminished. Long termstorage of the nanocrystallites under nitrogen, or another inert gas,can help maintain the quantum efficiency.

The PL of the CdTe nanocrystallites is temperature dependant. A reactionapparatus containing already synthesized 50 Å diameter CdTe nanocrystalgrowth solution was assembled in the fluorometer sample chamber andattached to a nitrogen source. The temperature of the system wasequilibrated for ten minutes and then a spectrum was obtained. FIG. 9displays the PL spectrum of the nanocrystallites at a range oftemperatures. The hysteresis in FIG. 9 is believed to be due to growthand/or decomposition that occurred during the latter half of theexperiment.

Transmission electron microscopy (TEM) provides valuable informationabout the size, shape, and distribution of the CdTe nanocrystal samples.Collecting a large population of measurements ensured statisticallymeaningful data. A JEOL 2000FX transmission electron microscopeoperating at 200 kV was used to obtain high resolution images of theCdTe nanocrystallites. 400 mesh copper grids with an ultralight coatingof amorphous carbon (Ladd) served as substrates. Solutions of CdTenanocrystallites were prepared by size selecting once with acetonitrile,washing the powder once with methanol, dissolving in THF, and dilutinguntil the absorbance at the first state was between 0.3-0.6 in a 1 cmcuvette. One drop of this solution was placed onto a carbon grid and,after 10 seconds had elapsed, the excess solution was wicked away with atissue. An objective aperture was used to improve contrast while stillbeing able to image the (111) lattice spacing, the most intense ring inthe diffraction pattern. Measurements were performed on images takenbetween 210,000-410,000× magnification. Microscope magnificationreadings were calibrated using the d-spacing of the (111) planesmeasured by X-ray diffraction (3.742 Å). The instrumental magnificationreading was found to be systematically low in this range; allmeasurements given herein are multiplied by 1.15 to reflect accuratevalues.

CdTe samples were carefully size selected once with acetonitrile asdescribed in U.S. application Ser. No. 08/969,302, incorporated hereinby reference. The powder was washed once with methanol and thendissolved in THF. Solution concentration was adjusted to deposit asubmonolayer coverage of nanocrystallites on the carbon substrate. TEMsamples prepared from THF contain few regions of closely aggregatedparticles and are superior to those laid out from hexane or pyridine.FIG. 10 shows a bright field TEM image of CdTe nanocrystallites from asample with an average size of 110±12 Å. This figure is representativeof the entire sample. Most of the nanocrystallites were approximatelythe same size and the same spherical shape. However, there was a sizedistribution, and some nanocrystallites had a distinctly prolate shape.As in the case of CdSe, the population of prolate nanocrystallites wasfound to increase with size. The inset of FIG. 10 shows neighboringnanocrystallites which were oriented so that the lattice fringesrepresenting (111) planes are visible.

For each examined CdTe size at least 250 nanocrystallites—in most casesapproximately 500—were measured in order to determine the averagediameter. FIG. 11 shows the histograms obtained for 8 different sizes.The standard deviations (10-14%) of nanocrystallites prepared by thesemethods are large in comparison to the best CdSe samples (<5%). FIG. 12plots the relationship between diameter and wavelength of the firstabsorbing state. The line graphs the third order polynomial which bestfits the data.

Powder X-ray diffraction (XRD) patterns provided the most completeinformation regarding the type and quality of the CdTe crystalstructure. The random orientation of nanocrystallites in a powder sampleensured that all possible crystal directions were probed. Estimates ofsize were also possible since particle diameter is inversely related,via the X-ray coherence length, to the peak width.

A Rigaku 300 Rotaflex diffractometer with a Cu anode operating at 60 kVwith a 300 milliamp flux was used to obtain powder XRD patterns. Sampleswere prepared by size selecting CdTe once with acetonitrile, washing thepowder three times with methanol, dissolving in the minimum amount ofTHF to produce a very concentrated solution, casting that solution ontoa silicon (001) substrate, and allowing the solvent to evaporate.Powders which would not dissolve in THF were dried under vacuum andstuck onto silicon substrates using RTV silicon adhesive.

FIG. 13 shows the experimental XRD patterns of three different size CdTesamples produced using HPPTTe. The peak positions match those of bulkcubic CdTe, which is represented by the stick spectrum at the bottom ofFIG. 13. Cubic CdTe is the thermodynamically stable bulk phase achievedbecause of high temperature nucleation and growth. Previous lowertemperature syntheses produced the less stable wurtzite phase of CdTe.The unassigned peak at 34° is believed to be due to a stacking faultwhich enhances the equivalent of the (103) direction in the wurtzitestructure.

Particle diameters were estimated using the Scherrer equation:$L = \frac{0.888\quad (\lambda)}{\Delta \quad \left( {2\quad \Theta} \right)\quad \cos \quad (\Theta)}$

where L is the coherence length (also known as the Scherrer length) ofthe X-rays, λ is the wavelength of the X-rays, Δ(273) is the FWHM inradians, and Θ is the angle of incidence. The coherence length L is amathematical construction that is related to real dimensions through thevolume average of the particle. For a sphere of diameter D therelationship works out to

D=(4/3)L

Using these equations and the FWHM of the (111) reflections gives sizes(45, 54, 81 Å) which are only slightly smaller than those obtained byTEM (47, 56, 92 Å). Since the Scherrer method actually measures thecoherence length of the X-rays, any crystal imperfections will cause thecalculated size to be smaller than the true size. The fact that the XRDsizes are close to the TEM size implies that the samples were verycrystalline, at least in the <111> directions.

Overcoating CdTe Nanocrystallites

While high quantum efficiency from non-overcoated or “bare” CdTenanocrystallites is very important in and of itself, it also affects theoutcome of subsequent protective shell growth attempts. Empiricalobservations from (CdSe)ZnS and (CdSe)CdS studies show that thisovercoating process generally has a multiplicative effect on the quantumefficiency of the starting material: the brightest (core)shell samplesgenerally come from the brightest nanocrystal cores. Semiconductor bandoffsets must be compared to determine which potential shell materialsprovide energy barriers for both the electron and hole. Since reliablevalues have not been published for nanocrystallites, bulk values can beused as a general guide. FIG. 14 shows the positions of the bandsrelative to the vacuum level for the II-VI zinc and cadmiumsemiconductors. ZnS and ZnSe appear to be suitable shell materialcandidates. Growth of ZnS or ZnSe shells onto a CdTe core produces acomposite material which is much more robust during processing andmanipulation. (CdTe)ZnS and (CdTe)ZnSe overcoated nanocrystallites havebeen synthesized using both a one-step and a two-step synthesis process.

One-step Synthesis of Overcoated Nanocrystallites

CdTe nanocrystallites were synthesized as described above. Once thecores had reached the desired size, the temperature was lowered to 200°C. The amounts of Zn and S precursors needed to grow a ZnS shell ofdesired thickness for each CdTe sample were determined as follows.First, the average radius of the CdTe nanocrystallites was estimatedfrom TEM or SAXS measurements. Next, the ratio of ZnS to CdTe necessaryto form a shell of desired thickness was calculated based on the ratioof the shell volume to that of the core volume, assuming a sphericalcore and shell and using the bulk lattice parameters of CdTe and ZnS.Equimolar amounts of diethyl zinc (ZnEt₂) and hexamethyldisilathiane((TMS)₂S) were added to 5-10 mL trioctylphosphine (TOP). The Zn and Sprecursor solution was added dropwise to the stirring CdTe reactionmixture over a period of 5-10 minutes. After the addition was completethe mixture was cooled to 90° C. and left stirring for several hours. 10mL butanol were added to the mixture to prevent the TOPO fromsolidifying upon cooling to room temperature. The overcoated particleswere stored in their growth solution to ensure that the surface of thenanocrystallites remained passivated with TOPO. (CdTe)ZnSe wassynthesized in the same fashion, with bis(trimethylsilyl) selenide((TMS)₂Se) serving as the selenium source. For both ZnS and ZnSeovercoats, the quantum efficiency of the PL was increased by 0-20%.

Two-step Synthesis of Overcoated Nanocrystallites

CdTe nanocrystallites were synthesized as described above. The amount ofnanocrystallites was determined by weight and/or optical absorbance. 20g TOPO was dried under vacuum (0.5 Torr) for 1 hour, then cooled to 60°C. under nitrogen. The CdTe nanocrystallites were dispersed in hexane orTHF, mixed with the TOPO, and the solvent subsequently removed undervacuum. The amounts of Zn and S precursors needed to grow a ZnS shell ofdesired thickness were determined as follows. First, the average radiusof the CdTe nanocrystallites was estimated from TEM or SAXSmeasurements. Next, the ratio of ZnS to CdTe necessary to form the shellwas calculated based on the ratio of the shell volume to that of thecore volume, assuming a spherical core and shell and using the bulklattice parameters of CdTe and ZnS. Equimolar amounts of diethyl zinc(ZnEt₂) and hexamethyldisilathiane ((TMS)₂S) were added to 5-10 mLtrioctylphosphine (TOP). The Zn and S precursor solution was addeddropwise to the stirring CdTe reaction mixture over a period of 5-10minutes. After the addition was complete the mixture was cooled to 90°C. and left stirring for several hours. 10 mL butanol was added to themixture to prevent the TOPO from solidifying upon cooling to roomtemperature. The overcoated particles were stored in their growthsolution to ensure that the surface of the nanocrystallites remainedpassivated with TOPO. (CdTe)ZnSe was synthesized in the same fashion,with bis(trimethylsilyl) selenide ((TMS)₂Se) serving as the seleniumsource. For both ZnS and ZnSe overcoats, the quantum efficiency of thePL was increased by 0-20%.

Alternative Size Selection Procedure for CdTe Nanocrystallites

A second size selective precipitation method allows narrower sizedistributions to be obtained. For example, size distributions of lessthan 10% RMS deviation can be prepared. CdTe nanocrystals are isolatedin air by a modified size selective precipitation. After synthesis ofCdTe nanocrystals described elsewhere, the flask is cooled to ˜60° C.and mixed with 10 mL of trioctylamine and 10 mL of tetrahydrofuran(THF). Methanol or acetonitrile is added to the mixed solution until themixture becomes turbid. Size selected CdTe nanocrystals can be obtainedas a precipitated powder after centrifugation. Size selected CdTenanocrystals are moderately soluble to hexanes and extremely soluble toTHF. Trioctylamine is added to maintain high quantum efficiency of PLduring the size selection procedure as well as to suppress phaseseparations. It should be used immediately before the size selectionprocedure to avoid etching of the nanocrystal surface. After sizeselection, typical FWHM of PL spectrum is 35 nm. Spectra with FWHM aslow as 20 nm can be obtained.

Photoluminescence of CdTe Nanocrystals

FIG. 15 shows the typical room temperature photoluminescence (PL)spectra of CdTe nanocrystallites, which span the optical spectrumranging from 580 nm to 770 nm. The PL quantum efficiency of thesesamples range from 40% to 65% with CdTe nanocrystallites emitting around640 nm having the highest quantum efficiency. The quantum efficiencybecomes lower as the size of CdTe nanocrystallites gets smaller orlarger from the medium size. Full width half maximum (FWHM) of eachspectrum falls in the range of 45 nm to 70 nm before the size selectionprocedure. After size selection, the FWHM of each emission spectrumdrops to 35 nm. The spectra shown in FIG. 15 were obtained using CdTenanocrystallites having diameters of 4.0 nm, 4.5 nm, 4.8 nm, 5.2 nm, 5.8nm, 6.2 nm, 7.7 nm, 9.1 nm, 11.9 nm.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the methods and products described herein primarily related toCdTe nanocrystallites. However, it will be apparent to those skilled inthe art that these methods and products can be extended to form ZnTe,MgTe, HgTe, and alloys of all of these tellurides. Accordingly, otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A population of nanocrystallites comprising aplurality of nanocrystallites, each nanocrystallite including: ananocrystalline core comprising MTe, wherein M is selected from thegroup consisting of Cd, Zn, Mg, and Hg; and an overcoating of asemiconductor material on a surface of the core, wherein the pluralityof cores is monodisperse.
 2. The population according to claim 1,wherein each nanocrystallite photoluminesces with a quantum efficiencyof at least 20%.
 3. The population according to claim 1, wherein eachnanocrystallite photoluminesces with a quantum efficiency of at least40%.
 4. The population according to claim 1, wherein eachnanocrystallite photoluminesces with a quantum efficiency of at least60%.
 5. The population according to claim 1, wherein eachnanocrystallite photoluminesces with a quantum efficiency of at least₇0%.
 6. The population according to claim 1, wherein the plurality ofcores has a size distribution having a standard deviation no greaterthan 15% of a mean diameter of the population.
 7. The populationaccording to claim 1, wherein each core comprises CdTe.
 8. Thepopulation of claim 1, wherein each core photoluminesces at a wavelengthin the range of 435 to 800 nm.
 9. The population of claim 1 wherein eachovercoating comprises ZnS.
 10. The population of claim 1 wherein eachovercoating comprises ZnSe.
 11. The population of claim 1 wherein eachovercoating comprises CdSe.
 12. The population according to claim 1,wherein the FWHM is 45 nm or less.
 13. The population according to claim12, wherein the FWHM is 20 nm or less.
 14. The population according toclaim 12, wherein the FWHM is 15 nm or less.
 15. The populationaccording to claim 1, wherein the plurality of cores has a sizedistribution having standard deviation no greater than 10% of a meandiameter of the population.
 16. The population according to claim 1,wherein the core is a member of a population having a size distributionwith a standard deiation no greater than 5% of a mean diameter of thepopulation.
 17. A population of nanocrystallites comprising a pluralityof nanocrystallites, each nanocrystallite including: a nanocrystallinecore comprising MTe, wherein M is selected from the group consisting ofCd, Zn, Mg, and Hg, and an overcoating of a semiconductor material on asurface of the core wherein the plurality of cores is monodisperse, andeach core photoluminesces at a wavelength in the range of 435 to 800 nm.18. The population of claim 17 wherein each core comprises CdTe.
 19. Thepopulation of claim 17, wherein the plurality of cores has a sizedistribution having a standard deviation no greater than 10% of a meandiameter of the population.
 20. The population of claim 17, wherein theplurality of cores has a size distribution having a standard deviationno greater than 5% of a mean diameter of the population.
 21. Thepopulation of claim 17, wherein each overcoating comprises ZnS.
 22. Thepopulation of claim 17, wherein each overcoating comprises ZnSe.
 23. Thepopulation of claim 17, wherein each overcoating comprises CdSe.
 24. Thepopulation of claim 17, wherein each nanocrystallite photoluminesceswith a quantum efficiency of at least 20%.
 25. The population of claim17, wherein each nanocrystallite photoluminesces with a quantumefficiency of at least 40%.
 26. The population of claim 17, wherein eachnanocrystallite photoluminesces with a quantum efficiency of at least60%.
 27. A population of nanocrystallites comprising a plurality ofnanocrystallites, each nanocrystallite including: a nanocrystalline corecomprising MTe, wherein M is selected from the group consisting of Cd,Zn, Mg, and Hg, and an overcoating of a semiconductor material on asurface of the core, wherein the plurality of cores is monodisperse andeach core photoluminesces with a full-width at half maximum (FWHM) of 70nm or less.
 28. The population according to claim 27, wherein the FWHMis 45 nm or less.
 29. The population according to claim 27, wherein theFWHM is 20 nm or less.
 30. The population according to claim 27, whereinthe FWHM is 15 nm or less.
 31. The population of claim 27, wherein theplurality of cores has a size distribution having a standard deviationno greater than 10% of a mean diameter of the population.
 32. Thepopulation of claim 27, wherein the plurality of cores has a sizedistribution having a standard deviation no greater than 5% of a meandiameter of the popuation.
 33. The population of claim 27, wherein theeach nanocrystallite photoluminesces with a quantum efficiency of atleast 20%.
 34. The population of claim 27 wherein each core comprisesCdTe.